1. Technical Field
Embodiments of this disclosure generally relate to a fixing device employing an electromagnetic induction heating method and an image forming apparatus incorporating the fixing device.
2. Related Art
Image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction machines having two or more of copying, printing, scanning, facsimile, plotter, and other functions, may incorporate a fixing device employing an electromagnetic induction heating method to reduce startup time of the image forming apparatuses, thereby enhancing the energy efficiency.
For example, JP-2006-350054-A discloses a fixing device employing the electromagnetic induction heating method. The fixing device includes, e.g., a support roller (or a heating roller) serving as a heat generation body, an auxiliary fixing roller (or a fixing roller), a fixing belt stretched over the support roller and the auxiliary fixing roller, an induction heater serving as an induction heating unit and facing the support roller via the fixing belt, and a pressing roller to contact the auxiliary fixing roller via the fixing belt.
The induction heater includes, e.g., a coil (or an excitation coil) wound in a longitudinal direction of the induction heater, and cores (or coil cores) disposed around the coil. The induction heater faces and heats the fixing belt. The heated fixing belt heats and fixes a toner image formed on a recording medium conveyed between the auxiliary fixing roller and the pressing roller. Specifically, a high-frequency alternating current supplied to the coil forms an alternating magnetic field around the coil, which generates eddy currents on a surface of the support roller and its neighboring area. When the eddy currents are generated around the support roller serving as a heat generation body, the electrical resistance of the support roller leads to Joule heating of the support roller, thereby heating the fixing belt stretched over the support roller.
In such a fixing device employing the electromagnetic induction heating method, the heat generation body is directly heated by electromagnetic induction. Accordingly, compared to a typical fixing device using a halogen heater, the fixing device employing the electromagnetic induction heating method has a higher heat-exchange efficiency and therefore the surface temperature of the fixing belt can be increased to a desired fixing temperature more efficiently, that is, with less energy and a shorter startup time.
To obtain a uniform temperature distribution, JP-2007-264021-A provides an air gap between a side core and an arch core. Such a gap lengthens a magnetic path passing through a nonmagnetic material and therefore increases an amount of leaked magnetic flux. Consequently, the corresponding amount of heat generation is reduced. Therefore, the air gap is provided at a portion where the temperature is high. By contrast, the air gap is not provided at a portion where the temperature is low. Such a way of determining gaps between side cores and arch cores is usually employed to obtain a uniform temperature distribution.
FIG. 4 of JP-2007-264021-A illustrates an air gap 52 provided between an arch core 35b and a side core 33 with a core holder 44. The size of the air gap 52 is determined according to temperature distribution. However, the size determination involves a change to the size of the arch core 35b. Consequently, multiple arch cores 35b having different sizes are used to determine the gap size. Thus, the number of components increases and therefore production costs increases. In addition, cores obtained by sintering compressed ferrite powder contract in a sintering process. Hence, arch cores are likely to warp, causing a difference in size among the arch cores. Consequently, gaps may be created in different sizes, hampering uniform temperature distribution.